1. Field of the Invention
The present invention relates to a device and a process for producing single crystals of silicon (and similar materials) having little contamination or thermal oxidation induced stacking faults, thus enabling the production of wafers whose gate oxide films have excellent dielectric strength. This device and process are also suited for finely controlling the oxygen content of the pulled crystal.
2. Description of the Related Art
Single crystals that are produced according to the Czochralski process contain an appreciable amount of oxygen, which has been melted out of the quartz (SiO.sub.2) crucible, as the silicon melt reacts with the quartz crucible. Consequently, during repetitive heat treatment which occurs in the IC and LSI manufacturing processes, this oxygen tends to prevent the occurrence of slips and burrs. Furthermore, during the heat treatments at a temperature of approximately 1000.degree. C., oxide precipitates in the crystal aggregate to form stacking faults of high density and reduce the impurities in the surface layers of wafers cut from the crystal (a phenomenon known as intrinsic gettering).
FIG. 6 schematically illustrates a cross section of a current device and the pulling-process according to the Czochralski technique. The crucible 1 is comprised of a quartz vessel 1a on the inside and a graphite vessel lb on the outside. A heating element 2 is mounted outside the crucible 1, in which the melt 5, the charge material for the crystal melted by the heating element, is contained. A seed crystal 3 is lowered until it makes contact with the surface of the melt 5 and then is pulled upward to grow a crystal at its lower end. These parts and components are contained in a metallic vessel provided with a water cooling device, all of which constitute a whole device for producing single crystals.
During the course of the single crystal pulling process, inert gas (such as argon gas) of high purity is introduced into the metallic vessel 6 from above at the center, forming a gas flow 30. The gas flow 30 turns into a gas flow 31 containing both silicon monoxide (SiO) that has evaporated from the surface of the silicon melt 5 and carbon monoxide (CO) generated as a result of the silicon monoxide reacting at high temperatures with the graphite components such as the heating element 2, the graphite vessel 1b, etc. The gas flow 31 flows down along the outside and the inside of the heating element 2 to be discharged through the discharge ports 8.
Since the argon gas flow 31 in the metallic vessel 6 is turbulent and locally stagnant, silicon monoxide is deposited on the ceiling of the vessel 6 layer by layer or in particle forms. Fine particles or small blocks of the deposited silicon monoxide fall onto the surface of the melt 5, are incorporated in the boundary layer of the growing crystal and give rise to dislocations in crystal.
Another problem is that, the silicon melt is contaminated also, unless carbon monoxide is properly discharged. That is, the carbon monoxide incorporates into the single crystal, will induce lattice defects in the single crystal.
In order to effectively obviate these problems, two devices mentioned below have been proposed.
FIG. 4 illustrates schematically a pulling assembly (the first device) proposed by the U.S. Pat. 4,330,362: Kokoku No. 57-40119. This device is characterized by having an upper flat annular rim 7a projecting beyond the crucible edge and a joining piece 7b attached to this annular rim 7a and extending downwardly and conically from its inner edge, the joining piece 7b being 0.2 to 1.2 times as high as the crucible 1.
FIG. 5. illustrates schematically another pulling assembly (the second device) disclosed in Kokai No. 64-72984. This device is characterized by having a heat resistant and heat insulating cylinder 10 that extends downward coaxially surrounding the single crystal 4 being pulled and is tightly joined to the subvessel 6c at its junction with the metallic vessel 6. The device is also characterized by having a heat resistant and heat insulating annular plate 11, which closely rests on the upper end of the heat insulating component 12 and has an outside diameter nearly identical with that of the heat insulating component 12, the inside diameter of which tightly fits the above mentioned heat resistant and heat insulating cylinder 10.
The first and the second devices as mentioned above effectively increase the pulling rate by shielding the crystal pulled from heat irradiation and they prevent fine particles of silicon monoxide from falling into the melt, and suppress generation of the thermal oxidation induced stacking faults (OSF). However, these devices do not solve the problems of enhancing the dielectric strength of the oxide films of the wafers which are cut from the crystal. And without improving the dielectric strength, it is impossible to produce a small highly integrated semiconductor. In addition, these devices can not solve the problem of having an adverse effect on controlling the oxygen concentration in the crystal.
The exact mechanism of formation of faults, which deteriorate the dielectric strength of the oxide films, has not yet been clarified. It has been reported that the cores of faults in crystals, which constitute the origin of the defects in the dielectric strength of the oxide films, are formed during the crystal growth, the cores contracting at the high temperature stage (above 1250.degree. C.) and grown at the low temperature stage (below 1100.degree. C.) (See 30P-ZD-17, The Japan Society of Applied Physics Extended Abstracts, The 39th Spring Meeting, 1992). In short, the dielectric strength of the oxide films are known to depend on the thermal history immediately after crystal pulling.
In the first device as schematically illustrated in FIG. 4, the internal clearance of the circular truncated cone 7b located surrounding the crystal being pulled is as low as 0.2 to 1.2 times that of the crucible. Thus immediately after a crystal grows it is exposed to the low temperature atmosphere in the metallic vessel, and is cooled too rapidly to facilitate shrinkage of the defect cores. As the result the dielectric strength of the oxide films deposited on the wafers cut from the crystal is deteriorated.
In the second device as schematically illustrated in FIG. 5, since the heat resistant and heat insulating cylinder 10 is tightly joined to the water cooled metallic vessel 6 and the subvessel 6c at their junction, the internal surface of the heat resistant and heat insulating cylinder 10 is cooled by thermal conduction, a rapid cooling takes place at the high temperature stage immediately after crystal growing. Hence, immediately after the crystal grows only a slight shrinkage of the defect clusters occurs and as a result the dielectric strength of the oxide films are deteriorated.
While the oxygen concentration in the single crystal is required to be brought under control to the accuracy of .+-.0.75.times.10.sup.17 atoms/cm.sup.8 from the target value in order to effectively carry out gettering action about oxygen in the single crystals as mentioned before, it is strongly influenced by the state of the above mentioned argon gas flow.
The flow velocity of the argon gas (Vg) depends on the gas supply pressure (Pg), the gas flow rate (Qg), the gas passage cross section (Ag) and the internal furnace pressure (Pf). This relationship can be described by the formula (A) given below. EQU Vg=(Qg/Ag).times.(Pg/Pf) . . . (A)
This gas flow velocity (Vg) significantly influences the contamination of the single crystals by the silicon monoxide that evaporates from the surface of the melt.
Since in the first device illustrated in FIG. 4, the circular truncated cone 7b, the flat annular rim 7a, and the circular cylinder 13 are tightly connected to each other, the entire argon gas flow from the upper side of the pulling chamber forms a gas flow 31 that passes inside the joining piece 7b to the narrow gap between the bottom portion of the joining piece 7b and the surface of the melt 5, after which it flows downward along the inside and the outside surfaces of the heater 2.
Since, as mentioned before, the argon gas flow from the pulling chamber is turbulent, the influence of the upward flow of the silicon monoxide that evaporates from the surface of the melt 5 upon the argon gas flow is not uniform along the circumference of the joining piece 7b. Hence the outward flow of the argon gas at the lower end of the joining piece 7b is not circumferentially uniform, rather there is variation in the velocity of the the argon gas flow (Vg) between the lower end of the joining piece 7b and the surface of the melt 5 depending on the location along the circumference of the joining piece 7b.
When, in the first device shown in FIG. 4, the argon gas flow rate (Qg) is enhanced and the gas flow velocity (Vg) is increased, sufficient discharge of the silicon monoxide and the carbon monoxide can be obtained, thus preventing them from contaminating. However, since, the flow velocity (Vg) in the gap between the end of the joining piece 7b and the surface of the melt 5 subsequently increases, the local variation of the flow velocity increases, depending on the circumferential position. And variation in the surface temperature and the convection in the melt 5 is induced, making it difficult to control the oxygen concentration in the crystal within a certain range with desirable accuracy and reproducibility.
As the flow velocity (Vg) in the gap between the lower end of the joining piece 7b and the surface of the melt 5 increases, the surface of the melt 5 tends to vibrate, leading to a situation where it is impracticable to carry out drawing a dislocation-free single crystal.
On the other hand, when the argon gas flow rate (Qg) is reduced and the gas flow velocity (Vg) is lowered, the variation in the flow velocity (Vg) in the gap between the lower end of the joining piece 7b and the surface of the melt 5 decreases, hence control over the oxygen concentration is improved. However, decrease in the gas flow velocity (Vg) entails a certain loss in capability to discharge silicon monoxide and carbon monoxide, raising the problem of contamination of the silicon melt 5 by silicon monoxide particles and carbon monoxide.
The problem mentioned above exists not only with the first device but also with the second device illustrated in FIG. 5, and is a problem common to existing devices. In short, with existing devices there are difficulties in providing the dielectric strength of the oxide films for the highly integrated micro-semiconductor or in accurately controlling the oxygen concentration in the crystal.